Laser DMD
Direct-metal Deposition (DM) is a multi-layer metal cladding process where a fully dense clad layer is produced pixel by pixel by melting metal powder with a laser beam. Though a side-to-side laser and powder supply may be used, a concentric nozzle is preferably used. Relative position of the beam and the substrate is determined by computer numeric control with the instruction from a CAD/CAM software. Any three-dimensional shape can be fabricated directly from CAD/CAM data. This process can create parts by focusing an industrial laser onto a flat metal work piece or any geometric surface to create a molten pool of metal. A small stream of powdered metal is then injected into the melt pool to increase the size of the molten pool. In this process, the molten pool produced by the focused laser beam cools and solidifies, resulting in metal parts of superior quality and strength. By moving the laser beam back and forth and tracing out a pattern determined by CAD, the solid metal part is built—line by line, one layer at a time. Unlike traditional subtractive manufacturing processes for producing molds, the DMD process does not produce engineered scrap.
FIG. 1 is sketch of a closed-loop Direct Metal Deposition System. Surface finish on the order of 100 micron is possible. The closed-loop DMD process ensures close dimensional tolerance and enables fabrication of components with multiple materials. It can reduce the present 65-step hip joint fabrication process into 7 steps leading to enormous savings in labor cost and lead-time. DMD can be adapted directly to the digital data from MRI and X-ray Topography.
Breinan and Kear first reported fabrication of three-dimensional metallic components via layer by layer laser cladding in 1978 and subsequently a patent was issued to Brown et. al. in 1982. Recently, various groups are working world wide on different types of layered manufacturing techniques for fabrication of near net-shape metallic components. Integration of lasers with presently available multi-axis CNC machines and co-axial nozzle are the main innovation for fabrication of 3-Dimensional components.
Direct metal freeform fabrication has the ability to create full functional parts from metal powder. Closed-loop DMD provides advantages over existing solid freeform fabrication technologies. Closed loop optical feedback enables automatic fabrication of 3-dimensional parts without operator intervention, ensures consistent product quality and builds to a near net-shape accuracy of 0.007″-0.010″.
Since DMD collects the light from the interaction zone and uses it as a feedback to control laser power and other process parameters, the process can control the energy input into the process, dimensions, and the cooling rate. Consequently, the resulting part microstructure can be closely controlled.
Moreover careful control of the input laser energy and process parameters based on the optical signal from the process not only minimizes the energy input but also minimizes vaporization of metals and powder consumption. This results in a more environmentally friendly manufacturing process. The potential for developing novel industrial applications of DMD is currently limitless. Many groups are actively pursuing it commercially for direct fabrication of molds and dies, and the repair of these parts. Recently, DMD has been used to fabricate lightweight, high-stiffness titanium scaffolds for bio-medical applications. To date, directed material deposition processes have been used to fabricate fully functional metal prototype parts, fabricate and repair industrial tooling for plastic injection molding, die casting and forging, and to apply wear resistant and corrosion resistant surfaces to turbine blades.
Micro EDM
In the past decade, electrical discharge machining (EDM) technology has gradually evolved and become an important production process. The EDM can be categorized into two general configurations: wire and die-sinking EDM. The wire EDM process uses a traveling wire as the electrode to cut a groove in a workpiece. Continuous electrical sparks are generated between the wire and workpiece to remove the work-material. By using computer-controlled motors and precision slides, the thin wire electrode is guided in the X and Y directions to cut a precise, narrow groove in the workpiece. To manufacture cylindrical parts, an additional axis is added to rotate the workpiece. The diameter of the conventional EDM wire is 250 μm. The state-of-the-art micro wire EDM machine uses 25 μm diameter wires and current research efforts are pursuing the 10 μm diameter wire for EDM FIG. 2(a)).
The second configuration of EDM is the die-sinking. Instead of using a traveling wire, a electrode of any geometry can be used to “drill” unique geometries into the work piece as illustrated in FIG. 2(b). A unique capability of the die-sinking EDM process stems from the ability to move the electrode around the workpiece, like a milling tool, to erode the material and generate the desired geometry (FIG. 2(c)). The EDM cutting force is small and, therefore, the size of EDM electrode as well as the features generated can be very small, in the 20 to 100 μm range.
An extensive technological survey has been undertaken to determine the advances of the critical areas of the EDM technology in order to identify the technology drivers that will enable the development of the ultra precision, nano-scale dry EDM surface finishing process critical to the development of the net-shape tool manufacturing process. Research efforts to enhance and extend the EDM technology base include the improvement in EDM efficiency and surface finish by implementing the nano-scale spark erosion process and the development of dry or near-dry processing techniques. Luo has developed a model to study the random mobility of electrical sparks and concluded that the small gap size allows strong spark mobility. This model indicates the trend of. Kunieda et al. have been active in the research into the high precision finish cutting utilizing the dry wire EDM process.
The Panasonic MG-ED82W Micro-EDM station represents the state of the art in commercially available mEDM technology. The MG-EDS2W is a 3DOF die-sinking EDM system with an integrated cylindrical wire EDM station for the manufacture of micro-electrodes used by the die-sinking machining stage. Panasonic quotes the positioning accuracy of the MG-ED82W as 5 μm across 100 mm XY motion and indicates a maximum surface roughness, Rmax, of 0.1 μm when utilizing de-ionized water as the dielectric material and severely limiting the electrical discharge energy, or spark energy.
Thermal Error Compensation Technologies
Since 1991, breakthroughs have also been made in the area of thermal volumetric (planar) error compensation for machining and turning centers at the University of Michigan. The techniques have been applied to several different kinds of CNC machine tools. Experimental results show that the accuracy improvement is five to 10 times by instrument inspection, and three to five times from cutting tests.
Determining how many thermal sensors should be used and determining their optimum positions are two crucial problems in improving the accuracy and robustness of the thermal error component models. A statistical approach has been used for selecting better sensor locations in preliminary research. However, in a purely statistical approach, a large quantity of sensors and intensive experiments are needed. The concept of the optimization of the sensor location means that, if the sensors are put on these locations, then the fitted thermal error model will be most robust. In other words, the thermal error estimated using this model will give smaller estimation error under various working conditions. This concept is difficult to be represented mathematically since error modeling is involved. In this research a new criterion for the optimization of the thermal sensor location is presented. This criterion is based on the concept that if a good model can be fitted for different working conditions, then the relationship between the thermal errors and sensor temperatures should be uniform or near uniform for various working conditions.
FIG. 3 illustrates a typical sensor temperature-thermal error relation with one sensor. The two curves are related to two operation conditions, warm-up and cool-down, respectively. It is not a good sensor location because a single model cannot estimate the thermal errors of the two conditions accurately because the two error curves are not uniform.